We offer a new approach to studying the inspiratory muscles in humans based on "isokinetic" principles of muscle testing. Flow rate is kept constant while the inspiratory muscles contract with a maximum voluntary dynamic contraction (MVDC). The resulting pressure generated, measured at one lung volume, represents a single point on a "maximum pressure-flow" curve which is an analog of the force-velocity relationship. By repeating the test over a wide range of lung volumes and flow rates the entire maximum presure-flow-volume curve can be obtained which defines the mechanical characteristics of the inspiratory muscle pump. When MVDC's are repeated over several minutes with a constant breathing pattern, the pressure generated decays with time to a "sustainable pressure"; the rate of decay is a measure of endurance. The purpose of this study is: 1) to determine if the inspiratory muscles are maximally activated during these tests, 2) to determine if the decay in pressure during repeated MVDC's is due to a loss in activation or contractile force of the muscles, 3) to determine normal values for inspiratory muscle endurance using repeated MVDC's and compare them with more conventional submaximal dynamic testing, 4) to determine thoracoabdominal configuration and the role of the diaphragm during repeated MVDS'S, 5) to measure the effects of flow rate on inspiratory muscle endurance, and 6) to test a hypothesis to account for the effects of inspiratory flow rate on endurance. This approach can provide a practical, noninvasive method of defining the mechanical characteristics and endurance of the inspiratory muscle pump. It may be useful in identifying pulmonary disease patients at risk of inspiratory muscle failure, selecting patients who may benefit from respiratory muscle training or rest, and evaluating patients before and after various forms of rehabilitation, such as exercise. Furthermore, the methods are closely linked to basic physiologic tests of skeletal muscle function and provide experimental isolation of an important variable in contraction of the inspiratory muscles, namely flow rate, which has largely been ignored in previous studies. This feature will expand our knowledge of the factors which influence inspiratory muscle fatigue.